In the construction of integrated circuit devices, lead frames are used to provide electrical interconnection to a semiconductor circuit. Typically, the base metal of the lead frame is copper because of its high thermal conductivity. Other base metals include stainless steel and Alloy 42, which is an alloy of 42% nickel and 58% iron. In some instances, a nickel layer on the order of 100 microinches is formed over the base metal to prevent temperature driven diffusion of the copper to the surface of the lead frame. Corrosion products formed by copper diffusion, such as copper sulfides and oxides, will degrade the solderability of the lead frame and will reduce the shelf life of the final product.
The nickel layer, however, contains pores through which the corrosion products may migrate. A nickel layer thickness of at least 400 microinches would be needed to reasonably assure that no continuous paths through the nickel layer would be available for copper migration. Unfortunately, a thickness of this magnitude will crack when the leads are eventually bent to form the dual inline package (DIP) or surface mount integrated circuit (SMIC).
A layer of palladium (Pd) may be formed over the nickel layer. The palladium top surface may be thermosonically bonded to gold wire, providing a stronger bond than available with a silver lead end. Furthermore, palladium will not tarnish or oxidize in air, thus retaining a clean bonding surface indefinitely. The palladium layer, however, will produce a galvanic potential between the palladium layer and the copper base, drawing copper ions to the surface. This galvanic couple accelerates pore corrosion in the palladium plated lead frame, which results in oxides and sulfides and other reaction products of copper appearing on the lead frame surface. The oxides and sulfides and other corrosion products discolor the surface of the lead frame and degrade its solderability.
Therefore, a need has arisen to provide an integrated circuit lead frame with reduced galvanic potential for preventing surface corrosion.